What QL Field to use for Custom System Discounts

I want to add custom system discounts after Term/Volume Discounts but before Additional Discount.

Need to keep the functionality of Volume Discount Schedules using Price and allow users to use Additional Discount to manually discount against a unit price (%, Amt, Total, Unit).

What is the best way to accomplish this?

Do I need to use Special Price? Taking List Price, applying Term/Volume Discounts (calculated from Schedules) and push that into Special Price.

Would prefer not to use a Custom Special Price Type on every record if possible.

If I want the Additional Discount field to work properly, I don't think I can override Regular Price or Customer Price. If I want these custom discounts to apply after Volume Discounts, I can't apply it to List.